In the fill level measurement, microwaves are sent by means of an antenna toward the surface of a fill substance and the echo waves reflected on the surface received. An echo function representing the echo amplitudes as a function of distance is formed, from which the probable wanted echo and its travel time are determined. From the travel time, the separation between the surface of the fill substance and the antenna is determined.
In industrial measurements technology, dielectric rod antennas and horn antennas are regularly applied for the sending and/or receiving. Typically, a housing is used, which has a housing section, which possesses the geometry of a short-circuited hollow conductor.
Inserted in the housing section with the hollow conductor geometry is a coupling element, via which microwaves are sent and/or received from and by the housing section. The microwaves for the sending are produced by a remotely arranged microwave generator and transported via coaxial lines to a sending/receiving system. In the antenna, there occurs via the sending/receiving system a changing from supplied, wire-transported microwaves into microwaves, which propagate in free space, and vice versa.
For determining the fill level, all known methods can be applied, which enable relatively short distances to be measured by means of reflected microwaves. The most common examples are pulse radar and frequency modulated, continuous wave radar (FMCW radar).
(Published international Application), WO01/88488 A1 describes (see the present FIG. 1) a fill-level measuring device having a rod antenna 35. The rod antenna 35 includes a cylindrical hollow conductor 39, which is short-circuited on one end by means of a rear wall 37. The fill-level measuring device includes a circuit board 49, on which a microwave generator 45 is located. Circuit board 49 includes a section 51, which protrudes laterally into the rod antenna 35 through an opening 52 in the hollow conductor 39. Section 51 is arranged parallel to the rear wall 37 and spaced from such. The sending/receiving element 47 is arranged on the section 51 protruding into the rod antenna 35.
The separation between the sending/receiving element and the rear wall must lie in a certain value range. Such value range depends decisively on the wavelength of the microwaves. Disadvantageous in a fill-level measuring device known from the state of the art (see the present FIG. 1) is that, in the course of time or in the course of the casting of the housing, the circuit board can shift and the sending/receiving element can change its separation from the rear wall. This can lead to destructive interference and, thus, to low intensity, radiated microwaves, whereby the accuracy of measurement of the fill-level measuring device is degraded.